1. Field of the Invention
The present invention relates generally to mechanical engineering and more specifically, to a device for positioning the table in metal-cutting machine tools in a horizontal plane along two coordinate axes.
The invention may be used to best advantage for machining holes or curvilinear surfaces in workpieces using numerically controlled machine tools, such as, for example, jig-boring, jig-grinding or milling machines.
2. Description of the Prior Art
Known in the art is a device for positioning the table of a metal-cutting machine tool in the XOY horizontal plane, along two coordinate axes X and Y, which is incorporated in a numerically controlled jig-boring machine (cf., a textbook "Jig-Boring Machine 2431", Service Manual, Part 1, V/O "Stankoimport", Moscow, c.6, FIG. 4, pages 11, 13, 14, FIG. 9, in Russian). The prior art device comprises a table mounted on the machine base and a contrivance for setting the position of the table in the XOY horizontal plane, as the table is moved. When the table is moved, its adjacent sides are in parallel alignment with the X and Y coordinate axes in the horizontal plane. Additionally, the device includes two drives adapted to move the table along either of the coordinate axes in the horizontal plane, the drives being mechanically interlinked with the said contrivance, and two means for reading the length of table travel along each of the coordinate axes disposed in the same plane which are also mounted on the machine base.
In the aforementioned known device the contrivance for setting the position of the table during its traverse comprises V-ways mounted on the base, and a slide which is provided with lower and upper V-ways disposed at right angles to each other (in a crosswise manner). The slide is placed with its lower ways onto the base ways, whereas its upper ways support the table provided with ways mating with the upper V-ways of the slide. Both drives imparting movement to the table are accommodated in the slide.
The drive to impart table movements along one of the coordinate axes comprises an electric motor provided with a reducer, and a gear which is mounted on the output shaft of the reducer and engages with a toothed rack rigidly secured to the table. This drive actuates table rectilinear movements along the slide.
The embodiment of the drive imparting table movements along the other of the coordinate axes is similar to that described above, except for the toothed rack, which, in this case, is rigidly secured to the machine base. This drive actuates rectilinear movements of the slide with the table in a direction perpendicular to that of table movement over the slide. Thus, the table is movable in the horizontal plane along two coordinate axes.
Constructionally, the embodiment of the contrivance for setting the position of the table as the slide having V-ways, fails to provide error-free positioning of the table since neither the slide proper nor its V-ways offer sufficient rigidity and manufacturing accuracy.
Attempts to improve the slide rigidity by increasing its height increase the positioning error still further. Besides, the table movement drives are mounted on the slide, which generates a need for long, non-rigid kinematic trains to impart motion from the electric motors to the final members of the train, i.e., the table and the slide, whereby the dynamic characteristics of the table positioning contrivance are impaired.
The kinematic trains of the table and slide actuation drives can be reduced in length by the employment of high-torque d.c. electric motors whose rotational speed can be varied within a wide range, and recirculating ball screw drives. In this case, the kinematic train will only comprise a single gear drive required for increasing the torque imparted by the electric motor, and a preloaded ball screw drive. However, recirculating ball screw drives are too sophisticated to manufacture, which makes drives of this type rather expensive. Moreover, these drives fail to provide stable dependability in operation since they are highly sensitive to overloads as well as to pollution of oil by dust even in small amounts, and, what is more, the kinematic motion transfer train still remains too long.
The table and slide mount one means for reading the length of table movement in a single plane along one of the coordinate axes, whereas the slide and base support another means for reading the length of table movement in the same plane along another of the coordinate axes.
In the aforementioned prior-art device, either of the means for reading the length of table movement comprises a measuring scale and a photoelectric microscope whereby optical readings are converted into electric pulses which are fed to the machine numerical control unit.
It is evident from the foregoing explanations that a construction where V-ways are arranged cross-wise at right angles to each other, in combination with the means for reading the length of table movement, will inevitably cause a substantial increase in the overall dimensions of the device and present problems during its installation. Rectilinear movements of the table and slide are obtained from the electric motors whose rotary motion is converted into translational motion via rather long drive chains. Besides, in a number of cases two- or three-speed drives may be required for more accurate positioning of the final member, i.e., the table. To complicate matters, the separate drives must be arranged so as to be mounted on the slide, which increases the overall dimensions of the device still further. The automation of table positioning through the employment of a numerical control system also poses a severe problem because the means of conveyance (ball screw drives) of the drives fail to provide the necessary feedback. If a numerical control system is to be used, the device must be supplemented with two readout systems providing the necessary feedback, which is unfeasible without increasing the overall dimensions of the device. This offers the only satisfactory explanation of the current situation when most of numerically controlled machine tools suffer from being too bulky whereas numerically controlled machines of compact design are practically not available.